An acceleration sensor of this kind is described in U.S. Pat. No. 4,827,091. This known sensor comprises a cylinder made of a conductive material, a magnetized inertial member mounted in the cylinder so as to be movable longitudinally of the cylinder, a conductive member mounted at least on one end surface of the inertial member which is on a side of one longitudinal end of the cylinder, a pair of electrodes disposed at the one longitudinal end of the cylinder, an attracting member disposed near the other longitudinal end of the cylinder and a testing coil for testing operation of the inertial member. When the conductive member of the magnetized inertial member makes contact with the electrodes, these electrodes are caused to conduct via the conductive member. The attracting member is made of such a magnetic material that the attracting member and the inertial member are magnetically attracted towards each other.
In this acceleration sensor, the magnetized inertial member attracts and the attracting member. When no or almost no acceleration is applied to the sensor, the inertial member is at rest at the other end in the cylinder.
If a relatively large acceleration acts on this acceleration sensor, the magnetized inertial member moves against the attracting force of the attracting member. During the movement of the inertial member,. an electrical current is induced in this cylinder, to produce a magnetic force which biases the inertial member in the direction opposite to the direction of movement of the inertial member. Therefore, the magnetized inertial member is braked, so that speed of the movement is reduced.
When the acceleration is less than a predetermined magnitude, or threshold value, the magnetized inertial member comes to a stop before it reaches the front end of the cylinder. Then, the inertial member is pulled back by the attracting force of the attracting member.
When the acceleration is greater than the predetermined magnitude, or the threshold value, e.g., the vehicle carrying this acceleration sensor collides with an object, the inertial member arrives at the one end of the cylinder. At this time, the conductive layer on the front end surface of the inertial member makes contact with both electrodes to electrically connect them with each other. If a voltage has been previously applied between the electrodes, an electrical current flows when a short circuit occurs between them. This electrical current detects collision of the vehicle.
When the testing coil is energized, the inertial member is moved up to the front end of the cylinder to make contact with the electrodes. Therefore, the testing coil is used for testing operation of the member.
It was found by the inventors that if the temperature of the surroundings of the acceleration sensor using the cylinder made of oxygen-free copper rises, then the electric resistance of the cylinder increases considerably. This reduces the electrical current induced by the movement of the magnetized inertial member. As a result, the magnetic braking force applied to the inertial member becomes less than intended.
Conversely, if the ambient temperature drops, the electric resistance of the cylinder decreases considerably. The result is that the magnetic braking force produced by the electrical current induced by the movement of the inertial member becomes greater than intended.
Where the braking force or damping force applied to the magnetized inertial member varies greatly, the acceleration sensor detects accelerations with great errors.